Solvent extraction



Oct. 31, 1967 A, c. MCKINNIS soLvENT EXTRACTION Filed Dec. l, 1961 United States Patent Gilfice i@ Patented Oct. 31, 1967 3,356,476 SOLVENT EXTRACTIGN Art C. Mclinnis, North Long Beach, Calif., assigner to Union @il Company of California, Los Angeles, Calif., a corporation of California Filed Dec. 1, 1961, Ser. No. 156,372 8 Claims. (Si. 26S-674.1)

This invention relates to a solvent extraction method for separating diaromatic hydrocarbons from monoaromatic hydrocarbons in admixture therewith. More specically, the invention relates to such a method in which is employed as a selective solvent for the diaromatic hydrocarbons and a lower alkane, of a type hereinafter specified, is employed as a solvent for the monoaromatic hydrocarbons. The invention has particular utility for the recovery of diaromatic hydrocarbons such as naphthalene, alkylnaphthalenes, etc., from mixtures thereof with monoaromatic hydrocarbons such as alkylbenzenes or the like of substantially equivalent boiling points. In a preferred application, the subject method is employed as a means of recovering Z-methylnaphthalene in high purity from alkylnaphthalene heartcut fractions of certain dealkylation process recycle streams. The origin, identity and nature of these dealkylation recycle streams, as well as all other details essential to an understandinf7 of this aspect of my invention, will be revealed in the complete description of the invention to follow.

it is well known that certain hydrocarbon mixtures of petroleum origin, as exempliiied by heavy reformate thermally treated to effect dealkylation of said methylnaphthalenes and thus obtain naphthalene as a product. in carrying out dealkylation processes of this type, it has been discovered that l-methylnaphthalene dealkylates from two to live times faster than Z-methylnaphthalene with the result that there is a buildup of the latter isomer in process recycle streams. Because Z-methylnaphthalene is a potentially valuable by-product, and its presence in dealkylation process recycle streams is detrimental (because of its relative slowness to dealkylate and its consequent tendency to accumulate in the system), a means of isolating it as a high grade (e.g., 95% pure) product would obviously serve a double purpose. However, in the past the primary interest has been in prod-ucing the maximum amounts of naphtnalene from the feedstock and until recently the possibility of removing Z-methylnaphthalene to accomplish the aforesaid twofold purpose was neither considered nor exploited, insofar as I am aware.

As a result of the present invention, the dealkylation of land 2-methylnaphthalene and dirnethylnaphthalene containing mixtures may now be conducted in such fashion as to effect conversion of a large proportion of the l-methylnaphthalene and the dimethylnaphthalenes to naphthalene (the latter lundoubtedly pass through the monomethylnaphthalene stage on their way to naphthaiene) while simultaneously effecting the removal of 2- rnethylnaphthalene from the system as a high grade byproduct.

There are a number of known solvent extraction procedures for isolating various components of hydrocarbon mixtures and numerous materials have been proposed for use as selective solvents in such procedures, typically representative of which are sulfur dioxide, furfural, diethylene glycol, nitriles, organic bases, etc. Such solvent extraction procedures have lbeen attempted with varying degrees of success on mixtures of aromatic and nonaromatic hydrocarbons to effect the separation of aromatics therefrom, and for this purpose sulfur dioxide,

one of the above-mentioned solvents, has proven to be particularly effective. However, sulfur dioxide is known tion with a lower alkane in the manner hereinafter solvents or identied in an equivalent fashion.

Briey, the method of this invention comprises a liquid-liquid extraction technique, employing the aforedioxide and lower alkane solvents and conducted at a reduced temperature, preferably within the range from about 20 to about `0 C. The reason for the low operating temperature is twofold. For one thing,

the boiling point of sulfur dioxide (-10 C.) is of a sufiiciently low order of magnitude to require operating temperatures within the above-noted range (where the petitive means of accomplishing the separation of mixtures of diaromatic and monoaromatic hydrocarbons.

There are many hydrocarbon mixtures, as,

lytic reforming of naphtha or This fraction normally boils above about 400 F. and contains from about 40 to about 8O percent by weight naphthalene and methylnaphthalenes, the remainder being either largely made up of monoaromatic compounds such as alkylbenzenes, tetralins, indanes, indenes, and the like,

fractionating and conversion operations, such as catalytic cracking, thermal cracking, catalytic reforming, catalytic cycle oil, etc., operations, also fit the above-described category. It is difficult to separate hydrocarbon mixtures such as these into diaromatic and monoaromatic fractions by conventional fractionation means, chiefly because of the boiling point proximity of the aromatic components. The present invention comprises a method of solvent extraction by means of which these separations are readily achieved. The hydrocarbon mixtures most amenable to the separation treatment of this invention are those of the above-indicated or similar types consisting of or containing substantially like-boiling diaromatic and monoaromatic hydrocarbons and boiling within the temperature range from about 400 to about 550 F. Such mixtures include not only those which boil throughout this entire range, but also those which boil over any portion thereof.

The expanding use of naphthalene for the production of dicarboxylic acids useful in manufacturing synthetic resins and bers has created a considerable interest in the recovery of alkylnaphthalenes from hydrocarbon mixtures, such as the aforesaid reformate fractions, as a feed source of dealkylation processes. Such dealkylation processes, as those skilled in the art appreciate, are normally of either catalytic or thermal type and they are designed to convert alkylnaphthalenes to naphthalene, a highly valuable starting material for the manufacture of phthalic anhydride. As such processes are conventionally carried out, a feedstock consisting of an alkylnaphthalene concentrate, boiling above about 430 F., from a reformate or cycle oil fraction which comprises alkylnaphthalenes and such non-naphthalenic materials are alkylbenzenes, alkyltetralins, alkylindanes, etc., is subjected to dealkylating conditions to obtain an effluent product which, after separation of normally gaseous materials such as hydrogen and low molecular weight hydrocarbons, is fractionally distilled to obtain: (1) a light gasoline fraction; (2) a naphthalene fraction; and (3) a heavy fraction comprising unreacted alkylnaphthalenes which, after the removal of heavy ends and polymers, is recycled to the reaction Zone. According to one mode of operation, the napthalene fraction is taken over a relatively wide range, for example, from about 400 F. to about 435 F., and the naphthalene is recovered therefrom in substantially pure form by low temperature crystallization, azeotropic distillation or solvent extraction means.

Where the dealkylation feedstock contains a significant quantity of 2-methylnaphthalene, as such feedstocks normally do, and the above-described procedure is followed, there is, as previously explained, a buildup of recycling 2- methylnaphthalene in the system. However, when the recycling stream containing the Z-methylnaphthalene is treated by the solvent extraction method of this invention the buildup of Z-methylnaphthalene is prevented and in addition that material is recovered in relatively pure form as a potentially valuable product. Briefly, in carrying out the dealkylation method in conjunction with my improved solvent extraction procedure, the liquid hydrocarbon dealkylation reactor effluent (obtained as indicated above and described in greater detail below) is distilled to obtain an effluent naphthalene fraction, a monomethylnaphthalene fraction, and a higher boiling fraction comprising polymethylnaphthalenes and other alkylnaphthalenes. The process feedstock is similarly distilled to obtain a monomethylnaphthalene fraction. Usually, the feedstock will be of fairly wide boiling range, comprising naphthalene and/ or alkylnaphthalenes of higher boiling point than the monomethylnaphthalenes, and in such event, the feedstock is distilled to isolate those components as fractions of elatively narrow boiling range.

In accordance with a preferred mode of operation, a relatively wide boiling range process feedstock is admixed with the aforesaid reactor effluent and the combined mixture is distilled to obtain the aforesaid fractions. The naphthalene fraction (whether it be obtained solely from the reactor eluent or from both the reactor effluent and the process feedstock, distilled together or separately) is treated by fractionation means such as azeotropic distillation, solvent extraction or crystallization and centrifuging to obtain essentially pure naphthalene and a filtrate or equivalent by-product stream which is useful as a heavy gasoline blending stock. The monomethylnaphthalene fraction (whether it be obtained by distilling the reactor effluent and the feedstock together or separately) is subjected to solvent extraction according to the method of this invention to obtain a ratnate product of non-naphthalenic hydrocarbons and an extract product of substantially pure Z-methylnaphthalene. The Z-methylnaphthalene extract product is separated as a by-p-rod-uct of the overall process. The raffinate product is admixed with the above-identified higher boiling alkylnaphthalene fraction (whether it be obtained solely from the reactor eluent or from both the reactor eflluent and the process feedstock, distilled together or separately) as feed to the dealkylation reactor, or otherwise disposed of.

Within the dealkylation reactor, the hydrocarbons are contacted with an alkalinized catalyst comprising cobalt and molybdenum oxides, the contacting being effected in the presence of added water and hydrogen at a temperature from about 950 to about 12.50 F. and at a pressure of about 600-1500 p.s.i.g. The effluent stream from the reactor is cooled and condensed to separate hydrogen and normally gaseous hydrocarbons, the water is decanted from the liquid phase, and the hydrocarbon phase is distilled as previously described. It has been found that by following the foregoing procedure it is po-ssible to convert to naphthalene as much as 85 percent or more of the l-methylnaphthalene and alkylnaphthalenes boiling above the monomethylnaphthalenes, while converting as little as 35 percent or less of the 2-methylnaphthalene, and at the same time recovering as much -as 60 percent yor more of the unconverted Z-methylnaphthalene lin substantially pure form. For more detailed descriptions of dealkylation procedures in which the solvent extraction method of this invention can be incorporated, see copending U.S. patent application Ser. No. 149,100, to Schaeffer and Stiles, filed Oct. 3l, 1961, and now issued as U.S. Patent No. 3,244,- 759, and copending U.S. patent application Ser. No. 117,553, filed June 16, 1961, now abandoned, to Schaeffer and Stiles.

As will be clear from the above, a principal object of my invention is to provide a solvent extraction method for use in conjunction with dealkylation processes whereby Z-methylnaphthalene can be recovered as a high grade by-product.

It is another object of the invention to provide a practical solvent extraction method by means of which diaromatic compounds are readily separable from likeboiling monoaromatic compounds in admixture therewith.

A more specific object of the invention is to provide a practical solvent extraction method by means of which naphthalenic compounds are readily separable from mixture-s containing like-boiling monoaromatic compounds.

Other objects, features and advantages of my invention will become apparent to those skilled in the art as the description of the invention proceeds.

The separation of diaromatics from monoaromatics according to the method of this -invention is by its nature a separation of hydrocarbon fractions `according to degree of aromaticity. The degrees of aromaticity of organic compounds of equivalent boiling points depend upon the number of aromatic rings (benzene nuclei) in their respective molecules, the higher the number of such rings the greater the aromaticity of a given compound. Thus, diaromatic compounds, those having 2 aromatic rings per molecule, are considered to have a greater degree of aromaticity than the monoaromatic compounds which have molecular structures containing only l such ring. It makes little difference, insofar as degree of aromaticity is concerned, whether polyaromatic compounds have individual or condensed ring systems. Thus, diaromatics of individual ring systems such as biphenyl, diphenyl methane, etc., have roughly the same degree of aromaticity as do the dinuclear aromatics (diaromatics having condensed ring systems), such as naphthalene, alkylnaphthalenes, etc., at least insofar as amenability to extraction with sulfur dioxide is concerned.

Attention is now directed to the accompanying drawing which schematically illustrates, as a preferred method for the practice of my invention, a continuous countercurrent solvent extraction process employing sulfur dioxide in conjunction with a lower alkane wash solvent as a twophase solvent system. A feed stream containing diaromatic VThe extract phase from solvent extraction column 1, normally containing from about l0 to about 30 percent diaromatic hydrocarbons and from about 70 to about 90 percent sulfur dioxide (and containing a -minor amount of lower alkane as previously noted), is passed through line 7 and into evaporator 23 wherein the sulfur dioxide is evaporated therefrom leaving as a bottoms product, the diaromatic-enriched extract from the feedstock. The evaporated sulfur dioxide is recirculated to solvent extraction column 1 through compressor 24, wherein it is liquefied, and line 5 as shown on the drawing, and the diaromatic hydrocarbon bottoms is withdrawn through line 25. None of the components of the extract phase decompose at or near their normal boiling points. Furthermore, the boiling points of the extract components are typically of such magnitude and disposition as to permit evaporation of the extract phase at atmospheric pressure and, accordingly, evaporator 23 is preferably operated at about that pressure leve-l.

The foregoing discussion is predicated upon the assumption that the amount of lower alkane carried from solvent extraction column 1 in the extract phase is so small that its presence can be safely ignored in the subsequent treatment of that phase to recover sulfur dioxide therefrom. This assumption is not unrealistic but representative of common experience in the practice of my invention. However, in certain instances, because of higher carry-over or other reason(s), it might be unwise or undesirable to ignore the presence of the lower alkane in the extract phase. In such a case, it is a simple matter to remove the lower alkane from the extract phase, either by adjusting the temperature in evaporator 23 to a sufficiently high level to flash off the lower alkane, in which case it is withdrawn overhead in admixture with the evaporated sulfur dioxide, or by substituting a fractional distillation column for evaporator 23 and recovering an overhead of sulfur dioxide, a side-cut of lower alkane and a hydrocarbon bottoms product enriched in diaromatic hydrocarbons. In the former event (ash evaporation of lower alkane and sulfur dioxide concurrently) it is a relatively simple matter to separate the resulting vaporous mixture by fractional liquefaction (e.g., fractional compression or condensation) means. The ease with which such a separation can be made is apparent from the great boiling point disparities between the lower alkanes suitable for use in my invention and sulfur dioxide.

Any lower alkane separately recovered from the extract phase from column 1 is liquefied and recycled for reuse in the system, discarded or otherwise disposed of. As an alternative to the separate recovery and disposal of the sulfur dioxide and lower 'alkane vapors from evaporator 23, or its equivalent, the vapors may be liquefied and thereafter recycled to the top of column 1 together, either as a homogeneous liquid solution or a two-phase liquid mixture. Whether or not the vapors liquefy as one phase or t-wo phases depends on such variables Yas the proportion of lower alkane present, the temperature of liquefaction, the pressure of liquefaction, etc.

As previously noted, the operating temperature of column 1 must be of a suiciently low order (preferably within the range from about -20 to about 0 C.) to maintain the low boiling sulfur dioxide in a liquid state. Consequently, the recycle sulfur dioxide from evaporator 23, or its equivalent, must at some stage prior to entry into column 1, or almost instantaneously thereafter, be cooled to a sufficiently low level to assure proper functioning of the system. There are a number of ways of accomplishing this well within the ability of those skilled in the art, exemplary of which are the use of cooling means such as chillers or the like in conjunction with or in place of compressor 24 and/or at other location(s) along the sulfur dioxide recycle route. In this connection,

Ait is pointed out that compressor 24- represents merely one possible means of liquefying the sulfur dioxide vapors from the extract phase. The use of other means to accomplish this liquefaction, such as, for example, refrigeration condensation means, is within the scope of this invention.

The bottoms product from evaporator 23 is recovered as the extract product of the process, or it can be circulated to further treatment, not shown, such as some form of purication treatment to remove traces of sulfur dioxide and/ or other contaminant(s) possibly present. Make-up sulfur dioxide is introduced into the system through line 27 and valve 29 as needed. Likewise makeup lower alkane is introduced into the system through line 11 and valve 9 as needed.

In addition to being highly selective toward aromatic hydrocarbons, sulfur dioxide exhibits high solvent power, that is, relatively small quantities of sulfur dioxide dissolve relatively large quantities of aromatics. More specically, sulfur dioxide proportions of from about 0.25 to about 3 parts (my preferred proportions) to about l part of hydrocarbon feedstock, on a weight basis, will normally manifest a solvent power of from about 5 to 35. The method of calculating solvent power values for purposes of this invention is set forth in Example Il following. The only criteria of significance for determining the proportions of lower alkane for use in this invention are those of practicality and cost, since any quantity of lower alkane will bring about the desired separation, at least to some extent. Preferred proportions based on these criteria, are typically from about l to about 5 parts by weight of lower alkane to l part by weight of hydrocarbon feedstock.

An important factor in arriving at optimum conditions of operation in solvent extraction processes is the rapidity with which the raffinate phase separates from the extract phase under various circumstances. By using the combination of sulfur dioxide and lower alkane solvents taught herein in the method of my invention, exceptionally rapid separation takes place between the phases at the temperatures of operation. The relatively fast phase separation is thought to be attributable, at least in part, to the fact that the sulfur dioxide has a relatively high liquid density (1.434), particularly by comparison with the relatively low density of the lower alkanes.

The lower alkanes suitable as wash solvents in the method of this invention are those which are liquid under the contemplated conditions of service. Thus, alkanes which are gaseous under such conditions as, for example, methane, ethane, etc., are obviously unsuitable. Higher alkanes, as, for example, tetradecane, eicosane, heptacontane, etc., are unsatisfactory in that they are either wholly or partly solid in the system, at operating temperatures, or `completely in solution in the sulfur dioxide, in which latter event there is only one liquid phase (rather than the required two phases) present. Pentane; isopentane; neopentane; hexane; Z-methylpentane; 3-methylpentane; 2,2-dimethylbutane; 2,3-dimethylbutane; heptane; octane; nonane; decane; etc., are exemplary of the lower alkanes suitable for use in my process. Generally, the lower alkanes preferred for use as wash solvents in the method of this invention are those saturated aliphatic hydrocarbons within the boiling range from about 95 to about 350 F., the preferred ones being the normal alkanes having carbon chain lengths of from 5 to l0, inclusive. Mixtures of suitable lower alkanes can be used as wash solvents within the scope of this invention. Specifically, my preferred wash solvent material is n-heptane because of its excellence of performance, ready availability and convenient boiling 'point (98.4 C.), the latter being of importance from the standpoint of wash solvent recovery from the solvent extraction product streams, particularly the rafnate phase stream. The term alkanes is employed herein in its conventional sense to denote the saturated aliphatic hydrocarbons.

The solvent extraction process of my invention can be carried out in various ways, the most common mode oi operation comprising the use of a spray, packed or bubble plate tower, wherein the hydrocarbon feed mixture is contacted by the streams of sulfur dioxide, usually owing countercurrently thereto, and lower alkane, usually flowing concurrently thereto.

Following are examples included for purposes of illustrating the invention. lt is emphasized that these examples are to be considered as illustrative only and not limitative of the scope of the invention.

Example I This example illustrates the selectivity of sulfur dioxide for diaromatics in the presence of monoaromatics when employed as a solvent in the method of this invention.

l ml. each of heptane and sulfur dioxide, and 3 ml. eac-h of tetralin and `alpha-methyinaphthalene were shaken together at 15 C. and the phases were allowed to settle and then separated. Tetralin is, of course, a monoaromatic hydrocarbon and alphamethylnaphthalene is a diaromatic hydrocarbon. The lower phase (sulfur dioxide) was found to contain 3.4 ml. of feed material comprising 43.8 volume percent tetralin. The upper phase (heptane) was found to contain 2.6 ml. of the feed material of which 54.8 volume percent was tetralin. This example clearly illustrates the selectivity of the sulfur dioxide for the alrha-metfhylnaphthalene in the feed mixture.

While it is true that the boiling points of tetralin and alpha-methylnaphthalene differ substantially, it is of interest to note that the tetralin is the lower boiling of the two. The significance of this will be apparent to those skilled in the art in View of the fact that the solubility of hydrocarbons in sulfur dioxide is generally an inverse function of boiling point, i.e., the lower the boiling point the greater the solubility. In lthe present example, in spite of the boiling point advantage of the tetralin over the alphamethylnaphthalene, insofar as solubility in sulfur `dioxide is concerned, the alpha-methylnaphthalene was still selectively extracted, thus emphasizing the ecacy of my method for separating diaromatics from monoaromatics in admixture therewith. By utilizing the dual solvent system of this example in a two-section column such las column 1 of the drawing, and including the proper number of stages in each section, relatively pure alpha-methylnaphthalene would be recovered as an extract product from a feed mixture like that here, and relatively pure tetralin would be recovered as a ratinate.

Repeating the above-described procedure, but substituting 20 ml. of sulfur dioxide for the 10 ml. each of heptane and sulfur dioxide, results in no separation of the feed mixture at all since both of its components are so soluble in sulfur dioxide that no two-phase system is formed. A like result occurs when instead of the ml. each of heptane and sulfur dioxide, l0 m1. of sulfur dioxide are employed.

Example II This example illustrates the selectivity of sulfur dioxide for diaromatic hydrocarbons in the presence of monoaromatic and nonarornatic hydrocarbons of like boiling range.

A 300 ml. quantity of liquid sulfur dioxide and 300 ml. of hexane are admixed with 100 ml. of a hydrocarbon fraction having a boiling range of 430-520 F., said fraction having been distilled from a mixture of FCC and TCC cycle oils, and being composed of 58.3 percent monoaromatics; diaromatics, including naphthalene, methyland dimethylnaphthalenes; alkyliudanes; alkylindenes; alkyltetralins; and the like. The balance of the hydrocarbon fraction (41.7%) is composed of paralns and naphthenes. The ratio of diaromatics to monoaromatics in the hydrocarbon fraction is 1.73z1. The mixture of sulfur dioxide, hexane, and hydrocarbon fraction is moderately agitated at 10 C. and allowed to settle into a raffinate and extract phase. The extract phase upon analysis shows an aromatic concentration of 96 percent which is found to be 80 percent diaromatics such as naphthalene, and methyland dimethylna'phthalenes, the balance being composed l@ of monoaromatics such as those named above. Thus, the ratio of diaromatics to monoaromatics is seen to have increased from 1.73 in the feed to 3.75 in the extract.

The above results clearly show that there is substantially greater selectivity of the sulfur dioxide for the diaromatics than for the monoaromatics present in the system. The selectivity factor determined from the results of this example is 3.7. The selectivity factor, or beta, as it is sometimes called, value is calcul-ated in accordance with the method set forth on page 39 of American Cyanamid Companys New Product Bulletin, Collective Volume H (December 1950). The selectivity factor is, as the term implies, a measure of the selectivity of a solvent toward a desired component, or components, of a mixture, the higher the value of the factor, the better the selectivity. As those skilled in the art will appreciate, the value of 3.7 is indicative of very good selectivity.

The solvent power ofthe sulfur dioxide, `calculated from the results of this example, is 5.1, an exceptionally high value and one which is indicative of good performance capability as a solvent for my purpose. The solvent power excellence of the sulfur dioxide is accented by the rather low ratio of sulfur dioxide to feed in the present example. Solvent power values are calculated by means of the formula: percent removal of bicyclicsXpercent improvement in bicyclics concentrationx 10-2. All percentage values representing or derived from component concentrations in this example are based upon weight concentrations.

Example III This example demonstrates continuous solvent extraction in accordance with the method of this invention.

Utilizing 100 grams (70 ml.) of sulfur dioxide as an extractive solvent for diaromatics and 100 grams (143 ml.) of n-octane as a wash solvent, a simulated continuous solvent extraction in three stages is carried out at atmospheric pressure and 15 C. on 60 grams (48 ml.) of catalytic cycle oil feed. The weight ratio of sulfur dioxide to feed is 1.46 to l. The feed is a hydrocarbon fraction which boils at a range between 445 F. and 500 F. The feed is of the following approximate composition (weight percent):

Parans 17 Naphthenes 2l Monoaromatics 20 Diaromatics 1 42 1. Consisting of methyluaphthalenes (14% on a tonal feed Weight basis) `and dimethyluaphthalenes (28% on a total feed weight basis).

An extract -phase containing 16 percent by weight of hydrocarbons and 84 percent by weight sulfur dioxide, `and a ranate phase containing 72 percent by weight noctane, 26 percent by weight hydrocarbons from the feed and 2 percent by Weight sulfur dioxide are produced. The sulfur dioxide-free extract contains about percent by weight naphthalenes. The sulfur dioxide is removed from the extract phase by flash evaporation. The recovery of naphthalenes from the feed is 78 percent.

This example, as does Example H, serves to illustrate the selectivity of sulfur dioxide as a solvent for diaromatics in the presence of monoaromatics in the dualsolvent technique of this invention, as well as the fact that the solvent power of sulfur dioxide toward diaromatics under the circumstances is relatively high. The use of additional stages in the method of this example results in a substantial increase in extract purity and yield of diaromatics. Thus, recoveries of from about 85 lto about 99 percent of the naphthalenes in the feedstock and extract product purities of about to about 99 percent diaromatics are attained by the use of such additional stages.

Example I V This example illustrates the selective extraction of di- 1 i. aromatics from a mixture thereof with monoaromatics in accordance with the method of this invention.

100 grams (70 ml.) of sulfur dioxide and 100 grams (161 ml.) of n-pentane is admixed at -15 C. with 75 grams (83 ml.) of a hydrocarbon fraction composed of 60 percent dimethyltetralins and 40 percent dimethylnaphthalenes. The ratio of solvent to hydrocarbon fraction is 1.33 to 1. A two-phase separation is conducted in which the dimethylnaphthalenes yare recovered as a concentrate in a sulfur dioxide-rich phase.

The sulfur dioxide-rich (or extract) phase contains about 28 percent dimethylnaphthalenes and about 72 percent sulfur dioxide. The recovery, or yield, of dimethylnaphthalenes achieved in this example is about 88 percent.

An increase in the number of stages increases the yield and purity of the extract product to values within the respective ranges set forth in the terminal paragraph of Example III.

it will be apparent that many Vmodifications of my process can be practiced simply by varying the lower alkane wash solvent, feed materials, quantities of ingredients, operating techniques, etc., within the permissible limits as taught herein. In addition to the previously described Z-methylnaphthalene recovery aspect of my invention as applicable to dealkylation processes, the process of the inventilon has general application for the preparation of feedstocks for dealkylation processes. Thus, a heavy reformate fraction containing alkylnaphthalenes and nonnaphthalenic materials can be treated in accordance with the invention to obtain an alkylnaphthalene concentrate which is thereafter dealkylated to form naphthalene by any of the conventional catalytic or thermal dealkylation processes.

A particularly significant aspect of this invention is its ready solution of the problem of separating diaromatic and monoaromatic fractions from hydrocarbon mixtures of high aromatic content such as, for example, hydrocarbon mixtures containing greater than about 50 percent by weight of diaromatic and monoaromatic compounds.

I claim:

1. A method for separating monomethylnaphthalene from a like-boiling hydrocarbon mixture comprising:

(1) continuously contacting said hydrocarbon mixture comprising monomethylnaphthalene and substantially similar boiling monoaromatic and nonaromatic hydrocarbons, at reduced temperature, in countercurrent relationship with liquid sulfur dioxide and in concurrent relationship with saturated aliphatic hydrocarbon liquid boiling within the temperature range yfrom about 95 F. to about 350 F., thereby forming an extract phase rich in liquid sulfur dioxide and containing a portion of said hydrocarbon mixture enriched in monomethylnaphthalene, and a rafnate phase containing substantially all of the remaining portion of said hydrocarbon mixture, said saturated aliphatic hydrocarbon liquid and a minor amount of said sulfur dioxide;

(2) evaporating said sulfur dioxide from said extract phase to form a sulfur dioxide overhead product and a hydrocarbon bottoms product enriched in monomethylnaphthalene;

(3) liquefying said sulfur dioxide overhead product to form liquid sulfur dioxide; and

(4) recycling said liquid sulfur dioxide to step (l).

2. The method of claim 1 in which step (1) is conducted at about atmospheric pressure and within the temperature range from about to about 0 C.

3. The method of claim 1 in which the ratio of said sulfur dioxide to said hydrocarbon mixture in step (1) is from about 0.25 to about 3 parts by weight of sulfur dioxide to 1 part by weight of hydrocarbon mixture.

4. The method of claim 1 wherein said hydrocarbon mixture is a monomethylnaphthalene fraction separated from the efiiuent product of a dealkylation process and wherein said monomethylnaphthalene consists essentially of Z-methylnaphthalene.

5. The method of claim 1 wherein said hydrocarbon mixture is a monomethylnaphthalene fraction separated from a mixture .comprising a dealkylation process feedstock and a dealkylation process eiuent product.

6. A method of extracting Z-methylnaphthalene from a like-boiling hydrocarbon mixture, comprising:

(1) continuously contacting said hydrocarbon mixture comprising Z-methylnaphthalene and similar boiling monoaromatic and nonaromatic hydrocarbons, at substantially atmospheric pressure and at a reduced temperature within the range of from about 20 C. to about 0 C., in countercurrent relationship with liquid sulfur dioxide as a solvent for the Z-methylnaphthalene, and in concurrent relationship with saturated aliphatic hydrocarbon liquid boiling within the range from about 95 F. to about 350 F. as a wash solvent, thereby forming an extract phase rich in sulfur dioxide and containing a portion of said hydrocarbon mixture enriched in 2-methylnaphthalene, and a raffinate phase containing substantially all of the remaining portion of said hydrocarbon mixture, substantially all of said saturated aliphatic hydrocarbon wash solvent and a minor amount of said sulfur dioxide;

(2) evaporating sulfur dioxide from said extract phase to form a sulfur dioxide overhead product and a first hydrocarbon bottoms product enriched in 2-methylnaphthalene:

(3) subjecting said raffinate phase to fractional distillation to form a saturated aliphatic hydrocarbon overhead product and a second hydrocarbon bottoms product enriched in nonaromatic and monoaromatic components;

(4) liquefying said overhead sulfur dioxide product from step (2);

(5) condensing said saturated aliphatic hydrocarbon overhead product from step (3) to obtain a saturated aliphatic hydrocarbon liquid',

(6) recycling said liquefied sulfur dioxide to step (1);

and

(7) recycling said condensed saturated aliphatic hydrocarbon liquid to step (1).

7. The method of claim 6 in which the ratio of said sulfur dioxide to said hydrocarbon mixture in step (1) is from about 0.25 to about 3 parts by weight of sulfur dioxide to 1 part by weight of hydrocarbon mixture, and the ratio of said saturated aliphatic hydrocarbon Wash solvent to said hydrocarbon mixture in step (1) is from about 1 to about 5 parts by Weight of solvent to 1 part by Weight of hydrocarbon mixture.

8. The method of claim 6 in which said saturated aliphatic hydrocarbon liquid is n-heptane.

References Cited UNITED STATES PATENTS 2,319,813 5/1943 Grosse et al 208-337 X 2,726,986 12/1955 Gross 208-338 X 2,727,854 12/1955 Brown et al. 260-674 2,758,141 8/1956 Findlay 260-674 3,003,006 10/ 1961 Francis 260-674 3,027,413 3/1962 Moy et al 260-672 OTHER REFERENCES Maxwell, Data Book on Hydrocarbons, p. 2 relied on, Van Nostrand Co., Inc., New York, New York 1950.

Nelson, Petroleum Refinery Engineering, Fourth Edition, pages 359-360 relied on, McGraw-Hill Book Cornpany, New York, N.Y. (1958).

Sachanen, The Chemical Constituents of Petroleum, pp. 219-23 relied on, Reinhold Publishing Corp., New York, N.Y., 1945.

DELBERT E. GANTZ, Primary Examiner. ALPHONSO D. SULLIVAN, Examiner.

C. E. SPRESSER, Assistant Examiner. 

1. A METHOD FOR SEPARATING MONOMETHYLNAPHTHALENE FROM A LIKE-BOILING HYDROCARBON MIXTURE COMPRISING: (1) CONTINUOUSLY CONTACTING SAID HYDROCARBON MIXTURE COMPRISING MONOMETHYLNAPHTHALENE AND SUBSTANTIALLY SIMILAR BOILING MONOAROMATIC AND NONAROMATIC HYDROCARCONS, AT REDUCED TEMPERATURE, IN COUNTERCURRENT RELATIONSHIP WITH LIQUID SULFUR DIOXIDE AND IN CONCURRENT RELATIONSHIP WITH SATURATED ALIPHATIC HY DROCARBON LIQUID BOILING WITHIN THE TEMPERATURE RANGE FROM ABOUT 95*F. TO ABOUT 350*F., THEREBY FORMING AN EXTRACT PHASE RICH IN LIQUID SULFUR DIOXIDE AND CONTAINING A PORTION OF SAID HYDROCARBON MIXTURE ENRICHED IN MONOMETHYLNAPHTHALENE, AND A RAFFINATE PHASE CONTAINING SUBSTANTIALLY ALL OF THE REMAINING PORTION OF SAID HYDROCARBON MIXTURE, SAID SATURATED ALIPHATIC HYDROCARBON LIQUID AND A MINOR AMOUNT OF SAID SULFUR DIOXIDE; (2) EVAPORATING SAID SULFUR DIOXIDE FROM SAID EXTRACT PHASE TO FORM A SULFUR DIOXIDE OVERHEAD PRODUCT AND A HYDROCARBON BOTTOMS PRODUCT ENRICHED IN MONOMETHYLNAPHTHALENE; (3) LIQUEFYING SAID SULFUR DIOXIDE OVERHEAD PRODUCT TO FORM LIQUID SULFUR DIOXIDE; AND (4) RECYCLING SAID LIQUID SULFUR DIOXIDE TO STEP (1). 